The present invention relates to shaft bearings and in particular, to bearing arrangements for use with a shaft arrangement having an inner rotatable shaft located within an outer shaft which is hollow and rotatable.
Rotating shafts generally require to be supported by a bearing arrangement during use, in order to control the alignment of the shaft. It is common practice to support a rotating shaft by means of ball or roller bearings. This necessitates the provision of a lubrication system, with attendant problems of size, weight and reliability, together with capital and maintenance costs. In order to obviate the need for a lubrication system, it has been proposed to support a single shaft by means of active magnetic bearings, which make use of magnetic interaction between the shaft, which acts as a rotor, and a stator which surrounds the shaft. The stator has pole pieces which carry energising windings so that the poles may be energised by passing electric current through the coils to create magnetic fields. The interaction of the magnetic fields between the stator and the rotor can be controlled to cause the rotor to levitate within the stator, so that the alignment of the rotor can be controlled without the need for mechanical bearings and lubrication.
The present invention provides a shaft arrangement having an inner rotatable shaft located within an outer shaft which is hollow and rotatable, there being a bearing arrangement for controlling the alignment of the inner shaft, the bearing arrangement having an electrical stator means outside the outer shaft and operable, in use, to generate magnetic flux which extends through the outer shaft to interact with the inner shaft to control the alignment of the inner shaft, wherein the outer shaft has a circumferential ring of regions alternately of relatively high and relatively low magnetic permeability, the ring being located between the stator means and the inner shaft so that flux is transmitted between the stator means and inner shaft through the ring region and is transmitted preferentially through the high permeability regions.
The stator means may include a ring of pole pieces and energising means operable to energise the pole pieces to create a magnetic field, the energising means being operable to commutate the magnetic field as the outer shaft rotates. Preferably the commutation causes the magnetic field to rotate at substantially the same rate as the outer shaft, whereby flux through the high permeability regions is substantially maintained as the outer shaft rotates. The energising means is preferably operable to produce a commutating waveform for energising the pole pieces in commutated manner, and is further operable to superimpose a control waveform on the commutating waveform, the control waveform serving to create a variable force on the inner shaft, to control the alignment of the inner shaft.
Preferably the stator means and the high permeability regions cooperate to form flux paths which pass substantially radially from the stator means to the inner shaft along radii which are circumferentially spaced around the shaft, and pass substantially circumferentially around the inner shaft, between the two radial positions. The alignment of the region boundaries is preferably skewed relative to the axis of the inner shaft.
Alternatively, the stator means and high permeability regions may cooperate to form flux paths which pass substantially radially from the stator means to the inner shaft along radii which are spaced axially along the shaft, and pass substantially axially along the inner shaft, between the two radial positions.
Preferably the ring of regions is created by forming a ring of slots in the material of the outer shaft, and locating inserts in the slots, the material of the inserts and the material of the shaft being of different permeability.
The stator means and ring may be arranged to allow a transverse force to be applied to the inner shaft, or to allow an axial force to be applied to the inner shaft.
The invention also provides a gas turbine engine having a shaft arrangement according to any of the foregoing definitions. Preferably, at least the inner rotatable shaft is a load transmitting shaft interconnecting a fan or compressor and a turbine assembly of the engine.